---
title: "How to Measure a Trapezoid Cushion: Depth Runs at 90°"
url: "https://cushionguide.com/answers/trapezoid-window-seat-depth-measurement/"
section: "The Cutting Table"
author: "Nate Kowalski, The Pattern Engineer of Detroit"
author_url: "https://cushionguide.com/masters/nate/"
published: "2026-07-24"
updated: "2026-09-24"
keywords: "how to measure a trapezoid cushion, trapezoid window seat depth, bay window cushion measurements, angled window seat cushion, trapezoid cushion template"
---

# Which measurement is the depth on an angled window seat?

*Most trapezoid mix-ups start with one line drawn on the slant. Here's the checklist, and the one bit of arithmetic that catches the slip.*

Also asked as: Which measurement is depth for a bay window?; Which dimensions do you need for a trapezoid?

> **The quick answer:** Depth is the straight line from the front edge to the back edge at 90 degrees, taken at the deepest point, never along the angle. For the listing's length, enter the total between the outer corners of the angled sections. Then add the long front edge, the short back edge and each angled side measured corner to corner, plus a photo and a drawing.

![Terracotta and ivory diamond-pattern bay window seat cushion on a limestone ledge overlooking cypress trees](https://cushionguide.com/media/bay-window-cushions/terracotta-diamond-bay-window-cushion-tuscan-stone-alcove-928.webp "A three-sided cushion cut to follow a stone bay, in a sun-proof outdoor weave that suits a window taking full afternoon light.")

## Key facts

- Depth is measured straight from front to back at 90 degrees, at the deepest point, not diagonally.
- Total depth is the most important measurement for a trapezoid template.
- Length in the listing is the total between the outer corners of the angled sections.
- Each angled side is measured as a straight line, corner to corner.
- A 40 in (102 cm) front, 30 in (76 cm) back and 17 in (43 cm) depth give angled sides of about 17.5 in, not 22 in (56 cm).
- A trapezoid order uploads two images: a photo and a drawing.
- Multi-piece orders list each cushion as length x width x thickness, e.g. 20 x 47 x 4 in (50 x 120 x 10 cm).

## The trapezoid checklist, in the order I'd tape it

1. Depth: straight from the front edge to the back edge at 90 degrees, at the deepest point. Not along the angle.
2. Length for the listing: the total between the outer corners of the angled sections.
3. Long base: the full front edge, corner to corner.
4. Short edge: the back edge, against the window.
5. Left and right angled sides: each a straight line, corner to corner.
6. Thickness, and for several pieces, each cushion as length x width x thickness.
7. A sketch with every number written on its own edge, plus a wide corner-to-corner photo.

## The 17.5-inch arithmetic check

The workshop checks that your numbers agree with each other. You can too. Take a seat with a 40 in (102 cm) front, a 30 in (76 cm) back and 17 in (43 cm) of depth. Each angled side steps out 5 in, so it measures √(17² + 5²) ≈ 17.7 in. The workshop's quick check calls it about 17.5 in. Either way, not 22 in (56 cm).

If your tape reads 22, one of the other three numbers is off, and a depth taken on the slant is the first one I'd re-measure. Angles are just lengths in disguise. The [trapezoid cushion calculator](https://cushionguide.com/studio/trapezoid-cushion-calculator/) runs the math, and the [trapezoid measuring Pattern](https://cushionguide.com/guides/trapezoid-cushion-measurements/) has the diagrams. Same rules for a [bay window cushion](https://cushionguide.com/cushions/bay-window-cushions/) cut as one trapezoid. What does your front edge measure, partner?

## Sending it so the numbers can be checked

1. **Draw the shape**: Pencil the trapezoid on paper and write each measurement on its own edge.
2. **Photograph the seat**: One wide photo, corner to corner, shows the angles better than close-ups.
3. **Upload both**: The trapezoid listing asks for two images, the photo and the drawing. That's all it needs.
4. **List extra pieces separately**: Write each cushion as length x width x thickness, for example 20 x 47 x 4 in (50 x 120 x 10 cm).

> **Tailor's margin:** On a trapezoid cushion, depth is the 90-degree line from front edge to back edge at the deepest point; a tape laid along the angle reads long.

> A 40 in front, 30 in back and 17 in depth give trapezoid sides of about 17.5 in; a 22 in side means one of the other numbers is wrong.
>
> — Nate

## Buyer questions

**A common question:** How do I measure my angled window seat so the numbers actually come out right?

**The workshop's answer:** Measure depth straight from front to back at 90 degrees, not diagonally, and each angled side as a straight line corner to corner. The workshop checks the geometry: a 40 in front, 30 in back and 17 in depth give sides of about 17.5 in, not 22 in. Sketch it with each number on its edge, or send a wide corner-to-corner photo.

**A common question:** What's the single most important number for a custom-shape template?

**The workshop's answer:** The total depth, straight from the front edge to the back edge at the deepest point, along with the exact long base and short back edge. For the trapezoid, give every edge plus the straight height between the parallel sides; for several pieces, list each as length x width x thickness.

**A common question:** What goes in the depth and length boxes for my bay window cushion?

**The workshop's answer:** Depth is the straight distance from the short angled sections to the long edge. Length is the total measured between the outer corners of the angled sections. Then upload the photo and the drawing, and nothing else is needed.

**A common question:** Should I take a little off my numbers so the cushion fits?

**The workshop's answer:** No. Send the exact size of the space and don't deduct anything. The workshop makes the cushion to your numbers and handles any fitting allowance in production; piping sits on the seams and doesn't change the size.

## Questions buyers ask next

### Why does depth matter more than the angled sides?

Depth is the height between the two parallel edges. With depth, the long front and the short back, the angled sides can be calculated and cross-checked, so a slanted depth reading throws every other number off. That's why it's taken at 90 degrees.

### My window seat angles are barely there. Do I still measure it as a trapezoid?

Yes. Even a slight angle makes the front and back edges different lengths. Take the same set: depth at 90 degrees, both parallel edges, each angled side corner to corner, and a sketch so the angles run the right way.

### Can I give trapezoid measurements in centimeters?

Yes, inches or centimeters both work. Pick one unit and use it for every edge, depth and thickness, and write it on the sketch. Multi-piece orders read cleanly as length x width x thickness, such as 19¾ x 47¼ x 4 in (50 x 120 x 10 cm).

## Related at the atelier

- [Do I need to measure the slanted sides of a trapezoid or bay window cushion?](https://cushionguide.com/answers/measure-slanted-sides-trapezoid-cushion/): Yes, measure both slanted sides of a trapezoid cushion, because they can differ, but the workshop treats them as a check, not the master dimensions.
- [Trapezoid Bench Cushion in Indigo: Contrast Piping in a Bay](https://cushionguide.com/readings/indigo-trapezoid-cushion-kitchen-breakfast-bay/): On a trapezoid bench cushion, contrast piping is worth it when the shape itself is the point.
- [Trapezoid Bench Cushion in Mocha and Teal, Near and Far](https://cushionguide.com/readings/mocha-trapezoid-bench-cushion-teal-piping-bay/): From ten feet, a trapezoid bench cushion reads as outline and color: here a teal welt draws the mocha seat against white cabinetry.
- [Trapezoid Bench Cushion for a Bay: Gray with Olive Piping](https://cushionguide.com/readings/light-gray-angled-bay-cushion-olive-piping-oak/): A trapezoid bench cushion that tapers almost to a point should be cut from a paper template of the ledge, not from tape measurements, because the narrow tip has no room to hide an error.
- [What happens if the slanted sides on my trapezoid drawing don't add up?](https://cushionguide.com/answers/trapezoid-cushion-measurements-dont-add-up/): On a trapezoid cushion, the top, bottom and straight depth decide the slanted side length, so the workshop recalculates your sides before cutting.
- [How to measure a trapezoid cushion so the numbers close](https://cushionguide.com/guides/trapezoid-cushion-measurements/): Trapezoid cushion measurements are the front edge, the back edge and the straight depth taken at 90 degrees, plus both slanted sides as a check.
- [One printable cushion measuring worksheet for any shape you order](https://cushionguide.com/guides/printable-cushion-measuring-worksheet/)

*± 1/4 in — Nate*

---

Written by Nate Kowalski (https://cushionguide.com/masters/nate/) for CushionGuide (https://cushionguide.com/about/). The site explains made-to-measure cushions and the partner workshop makes them; measurements are in inches first. This Markdown mirrors the page at https://cushionguide.com/answers/trapezoid-window-seat-depth-measurement/